Process of etching metal with ammonium persulfate with recovery and recycling



United States Patent 3,399,090 PROCESS OF ETCHING METAL WlTH AMMO- NIUM PERSULFATE WITH RECOVERY AND RECYCLING Frank E. Caropreso, Hamilton Square, Kenneth J. Radimer, Little Falls, and Bernard J. Hogya, Sayreville, N.J., assignors to FMC Corporation, New York, N.Y., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 451,635, Apr. 28, 1965. This application Sept. 26, 1967, Ser. No. 670,790

11 Claims. (Cl. 156-19) ABSTRACT OF THE DISCLOSURE A spent, aqueous ammonium persulfate etching soluuon, containing residual ammonium persulfate values and dissolved copper, cobalt, iron, nickel, zinc or magnesium is reactivated without substantial loss of ammonium persulfate values by cooling the solution to precipitate a double salt of the dissolved metal sulfate and ammonium sulfate and separating the double salt from the remaining solution; the reactivated etching solution thus recovered is used as the etchant per se or is reconstituted with additional ammonium persulfate values.

Cross reference to related applications This application is a continuation-in-part of application Ser. No. 451,635, filed Apr. 28, 1965, in the names of Frank E. Caropreso, Kenneth J. Radimer and Bernard J. Hogya.

Background of the invention (A) Field of the invention.This invention relates to aqueous ammonium persulfate etching solutions, and more specifically, to the reactivation of spent ammonium persulfate etching solutions containing residual ammonium persulfate values.

(B) Description of the prior art.-Solutions of peroxygen chemicals such as ammonium persulfate are commonly used to dissolve metals such as copper, cobalt, iron, nickel, zinc and magnesium. This is desirable, for example, in place of ordinary machining in order to remove specified amounts of these metals from surfaces of fragile or peculiarly shaped objects. A more widespread application of this technique is the production of printed electrical circuits. In this application a resist or mask in the form of the desired circuit is placed over the surface of a metal film, e.g., copper, laminated to a base, and the partially masked copper film is treated with the etchant. The copper area not covered by the resist is dissolved, while the copper covered by the resist remains to form the desired circuit.

Ammonium persulfate solutions are desirable in such applications because they do not generate obnoxious fumes, are easy to work with, and are relatively noncorrosive to certain common materials of construction, such as stainless steels. In use, the metal, e.g., copper, is dissolved in the nonmasked areas by the persulfate solution until the dissolution rate is sufiiciently low that it falls below commercially acceptable rates. The resulting spent etchant is then treated to remove the dissolved copper and is disposed of.

One serious problem that has arisen in using this process is that substantial amounts of persulfate are discarded in the spent etchant. It has not been possible to recover or make use of the remaining persulfate values in the spent etchant on a commercial scale; the mere addition of fresh ammonium persulfate to a spent etching solution to restore the original persulfate concentration does not yield an acice ceptable etching solution because such solutions give inferior and erratic etching.

A second problem is that the treatment of spent persulfate solutions to remove the dissolved metal entails an additional process step which adds to the expense of disposing of these solutions. The dissolved metal, e.g., copper, must be removed from persulfate solutions before they are sewered because of the toxicity of the metal values.

Objects of the invention It is an object of the present invention to treat spent persulfate etching solutions on a commercially acceptable basis to recover residual persulfate values and to eliminate the expense of removing dissolved metal in the solution and of disposing of the spent solutions.

It is a further object of the present invention to treat spent persulfate etching solutions to remove the products of the etching reaction selectively and to obtain reactivated solutions which yield consistent, commercially acceptable etching rates.

Summary of the invention We have found that spent, aqueous ammonium persulfate etching solutions which have been used to dissolve certain metals, namely copper, cobalt, iron, nickel, Zinc and magnesium, and which contain residual ammonium persulfate values can be reactivated by cooling these solutions to a temperature (preferably about 0 C.) sufficient to crystallize the dissolved metal sulfate values and ammonium sulfate values from the solution without crystallizing substantial amounts of ammonium persulfate, separating the crystallized solids from the remaining solution and recovering a reactivated solution suitable for etching; the ammonium persulfate values of the reactivated solution preferably are increased by adding fresh ammonium persulfate to the solution until a concentration of about 0.7 M to about 1.0M of ammonium persulfate has been reached.

Surprisingly, the reactivated ammonium persulfate solutions (with or without fresh ammonium persulfate addition) can be used to give high quality etching down to much lower residual persulfate values than can fresh, untreated ammonium persulfate solutions.

Description of the invention and the preferred embodiments In carrying out the present invention a fresh, aqueous 0.75 to 1.25 molar *(M) ammonium persulfate solution is made up (containing from about 171 g. to 285 g. of am monium persulfate per liter of water). This etching solution is then heated to temperatures of from about 35 to 46 C. and used to etch unmasked portions of either copper, cobalt, iron, nickel, zinc or magnesium. The etching can take place either by conventional immersion etching or spray etching. In the immersion etching process the masked metal coated workpiece is immersed in the solution for the amount of time required to etch the exposed metal surface. In the spray etching technique the persulfate solution is discharged from a spray nozzle under pressure and the spray impinges on the masked workpiece.

In practice, the spray etching technique is preferred because it permits shorter etching times and results in a better quality etch. This is due in large measure to the constant replacement of etchant in contact with the workpiece and to the removal of the dissolved metal-rich layer of etchant in immediate contact with the workpiece.

Etching is continued until the solution has been depleted of persulfate values and reaches a concentration of about 0.4 to 0.6 M ammonium persulfate. At this point, the solution is capable of further etching, but the etch rate and quality of etch diminishes and such solutions are normally discarded as spent solutions. In the following description, the invention will be illustrated by treating a spen ammonium persulfate solution used to etch copper. However, it should be understood that this same technique can be used to reactivate ammonium persulfate solutions used to dissolve cobalt, iron, nickel, zinc or magnesium, or alloys thereof, in the same manner.

This spent solution is treated in accordance with the present invention by cooling the solution, preferably to a temperature of about to 20 C., whereupon copper sulfate and ammonium sulfate values crystallize and precipitate from the solution. The exact temperature of cooling is not critical, but it is desired that it be sufficiently low to precipitate copper sulfate and ammonium sulfate values without precipitating substantial amounts of ammonium persulfate. A temperature of about 0 to C. is optimum for copper-containing etchants. In the case of nickel-containing etchants, temperatures as low as -8" C. can be used, while iron-containing etchants can be cooled to as low as -3 C. to effect crystallization.

The resulting sulfate-containing precipitate is separated from the solution by filtering or by centrifugal separators. During the separation care must be taken to remove residual solution from the precipitate in order to prevent loss of ammonium persulfate values. If desired, the precipitated crystals may be washed with water to recover any adhering persulfate values. The resulting reactivated solution, having a concentration of about 0.4 to 0.6 M. ammonium persulfate, can be used without further processing to obtain good etching of copper until the persulfate concentration of the solution reaches about 0.3 M. Thereafter, the quality of etching again diminishes.

An additional and preferred procedure is to reconstitute the treated etching solution, after cooling and crystal separation, by adding ammonium persulfate until the solution reaches a concentration of from about 0.7 M to about 1 M ammonium persulfate. Such a reconstituted solution can be used for etching until its ammonium persulfate content reaches about 0.3 M. Thereafter, it may again be treated by cooling to precipitate the copper sulfate and ammonium sulfate values and re-fortified with ammonium persulfate to reconstitute the etching solution.

When the reconstituted ammonium persulfate solution is intended for immersion etching, it is preferred that the solution be re-fortified with added ammonium persulfate up to a concentration of about 0.8 M ammonium persulfate. This in turn can then be used to etch copper until its persulfate concentration has decreased to 0.3

M. For use in immersion etching it is not recommended that the reconstituted etchant be re-fortified with ammonium persulfate beyond about 0.8 M for best quality etching. Reconstituted solutions containing greater than about 0.8 M ammonium persulfate do not give as high quality performance in immersion etching as do solutions containing no more than about 0.8 M persulfate. In some cases the reconstituted etching solution can be re-fortified with ammonium persulfate to concentrations of 0.9 M and still obtain good quality etching by increasing the temperature of use. However, such increase in temperature is not desirable since it may adversely affect the stability of the ammonium persulfate in solution. This limitation of ammonium persulfate concentration in the reconstituted solutions to 0.8 M need not be observed in solutions intended for spray etching where reconstituted solutions with 1 M concentrations of ammonium persulfate give excellent results.

After the reactivated solution (whether re-fortfied or not) has been used for etching and its ammonium per sulfate concentration has been decreased to about 0.3 M, it may be reconstituted repeatedly by coolin the solution to remove the ammonium sulfate and copper sulfate values and by re-fortifying the solution with ammonium persulfate.

In the above described embodiment of the invention, the spent etching solution was first cooled to precipitate ammonium sulfate and copper sulfate selectively prior to adding ammonium persulfate to reconstitute the solution. However, it is possible and in fact preferred to refortify the spent solution first by adding ammonium persulfate and subsequently by cooling the solution to crystallize and remove the ammonium sulfate and copper sulfate values. This latter technique is preferred because more copper sulfate and ammonium sulfate values are crystallized from solutions whose ammonium persulfate values have been increased prior to the cooling and crystallization step, than if cooling and crystallization is carried out on spent solutions containing depleted amounts of ammonium persulfate. The resultant, reconstituted solution contains less copper and ammonium sulfate and is capable of excellent etching at high dissolving rates.

Another technique which can be used to remove more copper from the spent solution during cooling and crystallization is to add ammonia or an ammonium salt such as (NH4)2SO4 or NH HSO to the spent solution prior to cooling. Any ammonium salt can be utilized that does not interfere with the etching or reactivation process, but the above salts are preferred to avoid introducing foreign ions in the etching solution. The amount of copper ions that can remain in the etching solution is a function of both the ammonium and sulfate concentration of the solution. As the concentration of ammonium ion in solution increases due to the added ammonia or ammonium salt, the amount of copper ion that can remain in solution decreases, and more precipitates during the cooling stage.

One embodiment of the invention that is contemplated to enable an ammonium persulfate solution to be used indefinitely for etching is carried out by continuously removing a portion of the etching solution, cooling said portion of the solution to precipitate sulfate insolubles, separating the sulfate insolubles from the supernatant solution, and recycling the supernatant liquor containing persulfate values back to the main body of etching solution. Simultaneously, ammonium persulfate is added continuously to the etching solution to maintain the persulfate concentration at the desired value, e.g., about 0.8 M to l M. The etchin solution can be used continuously with this cyclic crystallization and re-fortification procedure until foreign impurities build up in the solution to a point where the quality of etch is affected. Such a process obviates the need for replacing etching solutions and for disposing of the spent solution.

In general, the quality of etch obtained by using reconstituted persulfate solutions is about the same as that obtained with fresh ammonium persulfate solutions. Thus, the present technique does not necessitate any reduction in the high quality of etch current obtained when using ammonium persulfate etchants.

The ammonium persulfate etching solutions that can be treated by the present process can contain additives which enhance etching or modify etching characteristics. For example, the etching solution may contain additives such as sulfuric acid or phosphoric acid in addition to ammonium persulfate. It is within the contemplated use of the present invention that these solutions can be reactivated by the present invention in addition to aqueous ammonium persulfate Solutions per se.

The following examples are given to illustrate the present invention and are not deemed to be limiting thereof.

EXAMPLE 1 A series of runs were made for etching of photo-resisted, single-sided, one ounce/sq. ft, copper laminated, printed circuit test panels of equal size. In Run 1, 500 'ml. of a fresh, aqueous etching solution having an ammonium persulfate concentration of 1.11 M and 5 p.p.m. of HgCl as the activator was used to etch circuit test panels until the solution was approximately 50% depleted of its ammonium persulfate. The etching was conducted by immersing the test panels in the air agitated etchant maintained at a temperature of 38 to 39 C. The etch rates and quality of etch were determined periodically during the run. This run was considered as the standard and subsequent immersion runs were compared to it to determine the quality of etch and etching rates. An etch rate of above about 0.2 mil per minute is considered acceptable for commercial use.

The spent solution of Run 1 was cooled to a temperature of about 2 to 3 C. during which there was precipitated copper sulfate and ammonium sulfate values. These sulfate precipitates were removed from the mother liquor and sufiicient water was added to the mother liquor to re place that which had evaporated and that which had been removed as water of hydration of the sulfate crystals. The resulting solution was used as the etchant in Run 2 without any added ammonium persulfate.

In Run 2, the above solution was used to etch additional test panels as in Run 1 until the final etchant contained 0.34M ammonium persulfate. Thereafter, the etching solution was cooled and the copper sulfate and ammonium sulfate values were separated as previously described. The resulting solution was mixed with water to make up for the water lost by evaporation, and sufficient ammonium persulfate was added to obtain a solution having a concen-' tration of 0.82 M ammonium persulfate. This reconstituted solution was used as the etchant in Run 3.

In Run 3 additional test panels were etched with the reconstituted solution by immersion etching at 38 to 39 C. until the ammonium persulfate concentration fell to 0.39 M. Thereafter, this solution was treated by cooling to to 2 C. and removing the precipitated sulfates. The solution was reconstituted with additional ammonium persulfate until the total ammonium persulfate concentration was 0.82 M. This solution was used as the etchant in Run 4.

In Run 4 the reconstituted 0.82 M solution of ammonium persulfate was used to etch additional test panels by immersion etching at 38 to 39 C. until the ammonium persulfate concentration of the etchant reached 0.51 M ammonium persulfate. This etching solution was treated by cooling to 0 to 2 C. to precipitate ammonium and copper sulfate values, removing the precipitated sulfate valves, and adding sufficient ammonium persulfate to the remaining solution to increase the ammonium persulfate concentration to 0.78 M in the etching solution. This solution was used in conducting Run 5.

In Run 5 the above solution was used to etch additional panels by immersion etching at 38 to 39 C. until the ammonium persulfate concentration of the etching solution dropped to 0.43 M.

The persulfate content of the etching solution both before and after etching and before and after the crystallization step is shown in Table I. In addition, Table I indicates the amount of copper sulfate in the solutions prior to and subsequent to the crystallization, as well as the etching rate and quality of etch obtained with the various solutions used in the five runs. In all of these runs the precipitate contained copper sulfate and ammonium sulfate in a molar ratio of about 1:1. Thus, the copper sulfate content of the solutions as is reported in Table I also indicates the approximate molar concentration of ammonium sulfate in the etchant.

EXAMPLE 2 'Run A.A spent, aqueous persulfate etching solution having a concentration of 0.48 M ammonium persulfate and 0.50 M CuSO was treated by cooling the solution to about 2 C. during which copper sulfate and ammonium sulfate were precipitated and filtered from the remaining solution. This solution was then reconstituted by adding additional ammonium persulfate to a concentration of 0.82 M ammonium persulfate. The percentages of copper sulfate values and ammonium persulfate values removed from the solution in the precipitate were determined and are reported in Table II.

Run B.A spent, aqueous solution of ammonium persulfate etchant having a concentration of 0.48 M ammonium persulfate and 0.50 M copper sulfate was first fortified by adding sufficient ammonium persulfate to bring the concentration of the persulfate up to 0.82 M. This solution was cooled to about 2 C. during which copper sulfate and ammonium sulfate values were removed. The percentages of copper sulfate and ammonium persulfate values removed were determined as in Run A.

The content of ammonium persulfate and copper sulfate in the spent etching solutions and the amount of copper sulfate removed during the cooling and precipitation step in Runs A and B are set forth in Table II.

As will be observed from Table 11 only 51.4% of the copper sulfate in the solution was removed in Run A by first cooling the solution and later adding ammonium persulfate. In contrast, Run B wherein ammonium persulfate was added prior to cooling and crystallization, 63.5% of the copper sulfate in solution is removed. This illustrates that the procedure of Run B, wherein ammonium persulfate is added to the solution prior to cooling and removing sulfate values, is preferred, for in this way more sulfate and copper values can be removed.

EXAMPLE 3 Three runs were made in which photo-resisted, single sided, one ounce/ sq. ft., copper laminated, printed circuit test panels of equal size were spray etched. In Run 1, 4 gallons of 1.11 M ammonium persulfate solution was placed in a Center Circuits Spray Etcher Model 201, manufactured by Center Circuits, State College, Pennsylvania. The solution was heated to 41 to 46 C. and was sprayed from a nozzle onto the test panels. Test panels were etched with the solution until it had an ammonium persulfate concentration of 0.50 M. The etch rates and quality of etch were determined periodically during the run.

The spent solution of Run 1 was cooled to a temperature of 0 to 2 C. during which there was precipitated copper sulfate and ammonium sulfate in approximately a 1:1 molar ratio. These sulfate precipitates were removed from the mother liquor and sufiicient ammonium persulfate and water was added to produce an ammonium persulfate concentration of 1.0 M in the solution. This solution was used as the etchant in Run 2.

In Run 2 the reconstituted 1.0 M ammonium persulfate solution was used for etching additional test panels under the same conditions and temperatures as specified in Run 1 until the etchant reached a concentration of 0.52 M ammonium persulfate. It was then treated by cooling to 0 to 2 C. to precipitate copper sulfate and ammonium sulfate values. The precipitate was separated from the remaining mother liquor and sufficient ammonium persulfate was added to the remaining solution to obtain a 0.9 M ammonium persulfate concentration. This solution was used in conducting Run 3.

In Run 3 the above solution was used to etch additional test panels in the same manner as in Run 1 until the ammonium persulfate concentration of the etching solution dropped to 0.42 M.

The persulfate content of the etching solution both before and after etching and before and after the crystallization step is shown in Table III. In addition Table III indicates the amount of copper sulfate in the solutions prior to and subsequent to the crystallization, as well as the etching rate and quality of etch obtained in the three runs.

EXAMPLE IV A series of runs were made in which photo-resisted, single-sided, metal laminated printed circuit test panels were etched with a 1 M ammonium persulfate etchant containing 5 ppm. Hg as HgCl One etching solution used to etch nickel additionally contained 9% by volume HBF The metals etched were cobalt, iron, nickel, zinc and magnesium, respectively, in each run. The cobalt was in the form of Vicalloy assaying by weight, 52% Co, 35.5% Fe, 10% V and 0.5% Mn. The etching was conducted by immersing the test panels in 500 ml. solutions understood that, within the scope of the appended claims, of the air-agitated etchant, maintained at aternperature of the invention may be practiced by those skilled in the 38-39 C., until about 50% of the ammonium persulfate art, and having the benefit of this disclosure otherwise values were depleted. The magnesium sample only was than as specifically described and exemplified herein. etched at 25 C. The resultant, spent etchants were cooled What is claimed is:

to from -8 to +2 C. using a vacuum-type crystallizer 1. A process for reactivating an ammonium persulfate and crystals were precipitated from each of the soluetching solution which contains a dissolved metal selected tions. The crystals were then analyzed qualitatively to de from the group consisting of copper, iron, nickel, zinc termine their composition. The ammonium persulfate and and magnesium, said etching solution containing residual ammonium sulfate concentrations of the etchant solutions 10 ammonium persulfate, ammonium sulfate and the metal and the qualitative analysis of the crystals are set forth in sulfate corresponding to said dissolved metal, which Table IV. The etchant solutions that remained after cryscomprises cooling said solution to 8 to C. to tallization all gave good etch rates and etch characteristics. crystallize said metal sulfate values and ammonium sul- TABLE I Initial etching solution (M) Final etching solution (M) Etching characteristics Crystallization step Run Rates, mils of Percent removed (NHQzSzOs CuSOr" (NH4)2S2OB C1150; Cu/min. Quality Temp., C

(NHQ-gSgOs CUSO4 l Etch rates averaged over the entire period ofetching. 3 OuSOi concentration is that remaining after removal of crystallized 1 Etlrfzhting solution of Run 2 was not re-fortified with added ammonium sulfate values and removal of etchant samples for analysis. persu a e.

TABLE H fate values from the solution without crystallizing substantial amounts of ammonium persulfate, separating the Etchmgcomposmoum) crystalhzatlon Step crystallized solids from the remaining solution, and re- Percent remov d coverin a solution havin etchin ualities su erior t (NH;)2S208 ousot Te1np.,C. g I g g 0 (Ngmgsws sai ammonium persu fate etc mg solution prior to re- 0 4s 0 2 2 e 51 4 acnvation' 2 5 2. Process of claim 1 in which the residual ammonium persulfate concentration is at least about 0.4 M.

TABLE III Initial etching solution (M) Final etching solution (M) Etching characteristics Crystallization step u Rates, mils Q 1 t T 0 Percent removed NH S O CuSO 2 NH S O CuSO of Cu min. ua i y emp. 1)2 2 a 4 O2 2 s 1 (NHOZSEOB O 1 1.11 0.50 0.60 0 74 0-2 2.5 58.0 1.0 0 25 0.52 0. 7s 0 56 0-2 2.6 50.0 3 0.0 o 29 0.42 0.77 0 60 do l Etch rates averaged over entire period of etching. 2 011804 concentration is that remaining after removal of crystallized sulfate values and removal of etchant samples for analysis.

TABLE IV Spent etchant composition (molar Run Metal etched concentration) Crystalhzmg crystallized values temperature C. (NHOzSzOa (NH4l2SO4 Metal Sulfate 1 Cobalt (as Vicalloy) 0.5 0.4 0.4 0 COlDitfliJt sulfate, ammonium sulfate, iron SH 8. 8. 0.5 0.5 0. 5 3 Ferric sulfate, ammonium sulfate. 0.5 0.4 0. 4 0 Nickel sulfate, ammonium sulfate. 0. 5 0.5 0. 5 8 Zinc sulfate, ammonium sulfate. 5 Magnesium 0.5 0.3 0.3 0 Magnesium sulfate, ammonium sulfate.

When alloys, such as Vicalloy, have been etched as 3. Process of claim 1 in which said cooling is carried set forth in Example IV, Run 1, it is desired to remove out to temperatures of from about -8 to +20 C. both the dissolved cobalt and iron values from the per- 4. Process of claim 1 in which said cooling is earned sulfate etchant. In order to increase the amount of iron out to temperatures of from about 0 to 10 C. and other metals that are removed in the crystal pre- 60 5. Process of claim 1 in which the remaining solution cipitate, either ammonium sulfate or ammonium bisulfate is treated by adding ammonim persulfate thereto to incan be added to the etchant prior to cooling and crystalcrease the concentration of ammonium persulfate to lizing. This technique is particularly useful when it is about 0.7 M to about 1 M. desired to improve the effective removal of metals from 6. A process for reactlvatmg an ammonium persulfate the etchant which constitute only a minor portion of the etching solution which has been used to dissolve copper alloy that has been etched. The resulting metal values and which contains residual ammonium persulfate values, are precipitated either as metal sulfate values along with ammonium sulfate and copper sulfate values which comammonium sulfate values or as a complex ammonium prises cooling said solution to temperatures of about 0 metal sulfate salt. to 20 C. to crystallize said copper sulfate values and Pursuant to the requirements of the patent statutes, ammonium sulfate values from the solution without crysthe principle of this invention has been explained and tallizing substantial amounts of ammonium persulfate, exemplified in a manner so that it can be readily pracseparating the crystallized solids from the remaining ticed by those skilled in the art, such exemplification solution, and recovering a solution having etching qualiincluding what is considered to represent the best ernties superior to said ammonium persulfate etching solubodiment of the invention. However, it should be clearly tion prior to reactivation.

7. Process of claim 6 wherein said solution is cooled to temperatures of about to 20 C. to crystallize copper sulfate values and ammonium sulfate values from said solution without crystallizing substantial amounts of ammonium persulfate, separating the crystallized solids from the remaining solution, adding ammonium persulfate to said remaining solution to increase the concentration of ammonium persulfate to about 0.7 M to about 1 M and recovering a re-fortified ammonium persulfate etching solution.

8. Process of claim 6 wherein prior to cooling said solution, ammonium persulfate is added in amounts sufficient to increase the ammonium persulfate content of raid solution to a concentration of about 0.7 M to about 1! M.

9. A process for continuous etching with an aqueous, 0.3 to about 1.25 M ammonium persulfate solution which comprises contacting the etching solution with a metal selected from the group consisting of copper, iron, nickel, zinc and magnesium, and dissolving said metal, said solution containing residual ammonium persulfate, ammonium sulfate and the metal sulfate corresponding to said dissolved metal, removing a portion of the etching solution from the main body of etching solution, cooling said portion of etching solution to about 8 to +20 C.

to crystallize said metal sulfate values and ammonium sulfate values without crystallizing substantial amounts of ammonium persulfate, separating the crystallized solids from said portion of etching solution, returning said portion of etching solution free of said crystallized solids to the main body of etching solution, and adding ammonium persulfate values to the etching solution to maintain the effective etching concentration of said ammonium persulfate solution.

10. Process of claim 8 in which the solution is cooled to a temperature of from about 8 to +20 C.

11. Process of claim 1 wherein prior to cooling said solution, a salt selected from the group consisting of ammonium sulfate and ammonium bisulfate is added to said solution to improve removal of said dissolved metal during the subsequent cooling and crystallizing stage.

References Cited UNITED STATES PATENTS 1,589,610 6/1926 Marsh et al. 13413 2,408,934 10/1946 Kdellgren 23296 3,083,129 3/1963 Jones et al. 15619 3,216,873 11/1965 Jones 15614 25 JACOB H. STEINBERG, Primary Examiner. 

